1. Field of the Invention
The present invention relates to a container cap having good gas-barrier properties.
2. Description of the Background
Most containers for foods, industrial goods and others are required to have gas-barrier properties in order to prevent external gases such as oxygen and others from penetrating thereinto and to prevent active components and others existing therein from vaporizing out of them. Many methods are known for making container bodies have gas-barrier properties. For example, known are multi-layered containers comprising a gas-barrier material of gas-barrier resin, aluminium foil or the like, and glass containers. However, most caps, especially plastic caps for those containers are not made to have barrier properties, as their surface area to be kept in contact with contents is small. In the recent situation in the art, however, containers are required to have much better barrier properties, for which, therefore, it is much desired to provide caps having good barrier properties.
For making container caps have barrier properties, known is a method of adhering a gas-barrier material of aluminium or the like to the inner surface of caps while being laminated with an adhesive resin. However, gaskets of a multi-layered sheet of that type are generally produced by blanking the sheet having been prepared previously, and are therefore problematic in that their producibility is low and that the burrs formed in the blanking step are difficult to recycle as containing metals. Still another problem is that the technique of adhering the gas-barrier material to the inner surface of caps is extremely difficult in view of the poor adhesiveness between the gas-barrier material and caps and of the poor sealability of caps for containers.
Japanese Patent Application Laid-Open (JP-A) Hei-10-1579 (European Patent Laid-Open 761,752) discloses one example of gas-barrier gaskets. Briefly, it discloses a container packing (gasket) made of a multi-layered sheet comprising a resin composition layer and a polyethylene layer, in which the resin composition comprises a matrix phase of an ethylene-vinyl alcohol copolymer and domains of a block copolymer having aromatic vinyl polymer blocks and isobutylene polymer blocks, the domains being dispersed in the matrix phase. However, in the process of producing the gasket of such a multi-layered sheet, blanking scrap is inevitable. In addition, metal caps only are used in the Examples in the laid-open patent publication, and nothing is referred to therein relating to plastic caps.